In recent years, technology of interactive projection or interactive whiteboard has become more and more popular. Conventionally, the interactive pen and display device of the interactive projector utilize radio waves to transmit data. This kind of interactive pen usually has the function of wireless mouse. The user can select writing color by pressing the button of the interactive pen or using the interactive pen to click on the function menu shown on a frame.
However, conventionally, when the user wants to change the writing color, the user has to enter a function menu with several times of clicking. Besides, conventional interactive projection system normally only supports one writing color in the same writing region.